1. Field of the Invention
The present invention relates to a method for dry-etching a metal thin film or a chromium-containing half-tone phase-shift film and more specifically to a method and an apparatus for dry-etching such a metal thin film, which is applied to a process for preparing a chromium-containing half-tone phase-shift photomask which is used in fabricating, for instance, a semiconductor device as well as a method for dry-etching a chromium-containing half-tone phase-shift film which is used in a pattern-etching process for forming a fine pattern of a metal thin film such as fine electrode patterns (for a flat panel display (FPD) or the like) and color filters. In addition, the present invention also pertains to a chromium-containing half-tone phase-shift photomask which is provided with a pattern formed using such a dry-etching method and a method for preparing the photomask as well as a semiconductor circuit fabricated using such a photomask and a method for fabricating the semiconductor circuit.
2. Description of the Prior Art
As a photomask blank for chromium-containing half-tone phase-shift photomasks, there have been known, for instance, those having such a structure as shown in FIG. 1, which comprises a glass substrate a formed from, for instance, synthetic quartz glass; a light-shielding film or a half-tone phase-shift film b consisting of a thin film of a metal such as chromium or chromium oxide, formed on the surface of the substrate; and a resist layer c of a light-sensitive/electron-sensitive resin, which is formed on the light-shielding or half-tone phase-shift film. The glass substrate a may serve as a support for patterns and therefore, must have a variety of desired characteristic properties such as high transmittance, high uniformity, defect-free characteristics, resistance to washing and excellent flatness. In addition, the film b may serve as a light-shielding material for patterning or a phase-shift film and therefore, should satisfy the desired requirements for various properties such as etching controllability, uniformity, defect-free characteristics, resistance to washing, low stress and high adhesion to the glass substrate. Moreover, the resist layer c has a role as a film for forming a light-shielding film or a half-tone phase-shift film and accordingly, should have a variety of desired characteristic properties such as high-sensitivity/high resolution, resistance to etching, uniformity, defect-free characteristics and high adhesion to the light-shielding film or the film for forming a half-tone phase-shift film.
A photomask provided thereon with a fine electric circuit pattern has been prepared by wet-etching or dry-etching a chromium light-shielding or half-tone phase-shift film using a photomask blank having such a structure according to the electron beam patterning process or the laser beam patterning process. An example of such a mask-processing scheme is shown in FIG. 2. In this respect, the mask-processing scheme shown in FIG. 2 relates to a light-shielding film of chromium, but the processing scheme for the half-tone phase-shift mask is almost identical to that depicted on FIG. 2 and further details of the processing scheme for forming a half-tone phase-shift mask are described in Japanese Un-Examined Patent Publication (hereinafter referred to as xe2x80x9cJ.P. KOKAIxe2x80x9d) No. Hei 7-140635, the disclosure of which is hereby incorporated by reference.
In the wet-etching, there have recently been highlighted a limit in the dimension control due to the undercut and a limit in the verticality of the etched cross section, and the dry-etching technique has thus been widely used instead.
The dry-etching methods for preparing a photomask and the dry-etching apparatus for practicing the methods are described in, for instance, J.P. KOKAI No. Hei 6-347996, the disclosure of which is hereby incorporated by reference. In this dry-etching technique, a chromium film is etched using a gas comprising, for instance, chlorine gas to which oxygen gas is added, as a reactive ion etching gas.
Moreover, the dry-etching method for preparing a photomask of a chromium-containing film is disclosed in, for instance, Japanese Patent No. 2,765,065, the disclosure of which is hereby incorporated by reference. This patent discloses, in Examples, that when the chromium-containing film is dry-etched by this dry-etching method while using a resist film of a positive electron beam resist EBR-9 (which is available from Toray Industries, Inc.) as a mask and a mixed gas comprising 160 SCCM of chlorine gas, 40 SCCM of oxygen gas and 160 SCCM of wet air as a dry-etching gas, there is not observed any change in the etching rate of the electron beam resist film, while the etching rate of the chromium-containing film increases and the selective (or etching) ratio against the resist film is improved. As a result, the chromium-containing film can sufficiently be patterned by this dry-etching technique. In this connection, the wet air (160 SCCM) in the mixed etching gas comprises about 128 SCCM of nitrogen gas and about 32 SCCM of oxygen gas corresponding to the component ratio of nitrogen to oxygen in the air which is equal to 4:1.
In addition, the semiconductor circuit has recently become more and more finer and the size of the semiconductor circuit is increasingly reduced from 0.2 xcexcm to 0.15 xcexcm. For instance, in case of a semiconductor circuit fabricated using a conventional photomask, the dimensional error observed for the memory circuit portion is large as compared with that observed for the peripheral circuit portion in the memory circuit which comprises the memory circuit portion and the peripheral circuit portion, while such an error is also large even in the logic circuit and thus these errors may adversely affect the characteristic properties of the resulting circuit. For this reason, there has been desired for the development of a half-tone phase-shift photomask which permits the fabrication of a circuit whose dimensional difference between circuits within a semiconductor chip is as low as possible.
If a chromium or chromium oxide film as a light-shielding film or a half-tone phase-shift film is subjected to dry-etching using a chlorine-containing gas and if a pattern is formed on a plane at an almost uniform density, the film can be chromium-etched at an approximately uniform rate throughout the whole surface and accordingly, the dimensional control within a plane can be achieved to such an extent that the in-plane uniformity 3 "sgr" (3xc3x97the variance of (measured line widthxe2x88x92averaged line width)) ranges from 20 to 60 nm for the line width ranging from 1 to 2 xcexcm.
However, dense patterns (patterns whose area occupied by a resist is small) and coarse patterns (patterns whose area occupied by a resist is large) often coexist in the plane of a practical photomask and if the dry-etching technique is used for forming such a photomask, the etching rate of a chromium film is high at the densely patterned portion and low at the coarsely patterned portion. As a result, the dimensional difference within a plane reaches up to about 100 nm for a designed line width ranging from 1 to 2 xcexcm. A photomask having such a large dimensional difference within the plane cannot be used for the fabrication of, for instance, circuits having a higher integration density such as memory circuits, logic circuits and LSI circuits.
The Japanese Patent No. 2,765,065 described above does not relate to the solution of the foregoing problems, but relates to the improvement of the selective ratio of a chromium film to a resist film.
Accordingly, an object of the present invention is generally to solve the foregoing problems associated with the conventional technique or an improved dry-etching technique as a means for forming a fine pattern, which permits the reduction of the dimensional difference due to the coexistence of coarse and dense patterns within a plane, for instance, a dry-etching technique for manufacturing a chromium-containing half-tone phase-shift photomask. More specifically, it is an object of the present invention to provide a dry-etching method and a dry-etching apparatus, which permit the production of a high precision photomask by reducing the dimensional difference due to the coexistence of coarse and dense patterns within a plane.
Another object of the present invention is to provide a method for preparing a photomask using the foregoing dry-etching method and to provide a photomask thus prepared.
A further object of the present invention is to provide a method for fabricating a semiconductor device using the photomask and a semiconductor circuit fabricated by the method.
The inventors of the present invention have conducted various studies to achieve the foregoing objects, have found that even in the production of a chromium-containing half-tone phase-shift photomask in which dense patterns and coarse patterns coexist in the plane thereof, the use of a mixed etching gas comprising an oxygen-containing halogen gas such as an oxygen-containing chlorine gas (e.g., Cl2+O2), to which at least a hydrogen-containing gas (e.g., H2, hydrogen chloride (HCl) gas) is added, in the etching of the chromium-containing half-tone phase-shift film permits the achievement of in-plane dimensional control almost identical to that achieved for a mask in which patterns are formed in the plane at an almost uniform density, i.e., such in-plane dimensional control that the dimensional difference is not more than a half of that conventionally attained, for instance, 10 to 20 nm (0.010 to 0.020 xcexcm) and thus have completed the present invention on the basis of such a finding for the designed line width ranging from 1 to 2 xcexcm.
According to a first aspect of the present invention, there is provided a dry-etching method characterized by using, as an etching gas, a mixed gas including (a) a reactive ion etching gas, which contains an oxygen-containing gas and a halogen-containing gas, and (b) a reducing gas added thereto, in a process for dry-etching a metal thin film as a chromium-containing half-tone phase-shift film.
According to a second aspect of the present invention, there is provided a method for preparing a chromium-containing half-tone phase-shift photomask by performing a series of pattern-forming steps such as a step for forming a resist layer on a photomask blank, a step for exposing and patterning the resist layer, a developing step, a step for etching the photomask blank and a step for removing the resist layer and which is characterized in that patterns to be transferred onto a wafer are formed on the photomask blank for the chromium-containing half-tone phase-shift photomask according to the dry-etching method described above to thus give a photomask.
According to a third aspect of the present invention, there is provided a chromium-containing half-tone phase-shift photomask which is prepared through a series of pattern-forming steps such as a step for forming a resist layer on a photomask blank, a step for exposing and patterning the resist layer, a developing step, a step for etching the photomask blank and a step for removing the resist layer and which is characterized in that patterns to be transferred onto a wafer are formed on the photomask blank for the chromium-containing half-tone phase-shift photomask according to the dry-etching method described above to thus give a photomask.
According to a fourth aspect of the present invention, there is provided a method for manufacturing a semiconductor circuit which comprises the steps of transferring the patterns formed on the chromium-containing half-tone phase-shift photomask according to the third aspect of the invention on a wafer on which a light-sensitive material is coated, developing the light-sensitive material to form resist patterns on the wafer, or to manufacture a semiconductor circuit which comprises coexisting coarse and dense patterns corresponding to the resist patterns.
According to a fifth aspect of the present invention, there is provided a semiconductor circuit which has a circuit comprising coexisting coarse and dense patterns corresponding to the resist patterns formed by transferring the resist patterns formed on the chromium-containing half-tone phase-shift photomask according to the third aspect of the invention on a wafer on which a light-sensitive material is coated and then developing the light-sensitive material.
According to a sixth aspect of the present invention, there is provided a dry-etching apparatus used in dry-etching a metal thin film as a chromium-containing half-tone phase-shift film, which is provided with a sequencer for establishing dry-etching conditions, wherein the metal thin film is a chromium-containing half-tone phase-shift film consisting of a chromium film, a chromium oxide film, a chromium nitride film, chromium oxynitride film, chromium fluoride film or a laminated film thereof; wherein if an etching gas used consists of chlorine, oxygen and hydrogen gases, the relative flow rates of these gases as expressed in terms of % by volume range from 66 to 46, 17 to 11 and 18 to 41% by volume, respectively, or if an etching gas used consists of chlorine, oxygen and hydrogen chloride gases, the relative flow rates of these gases as expressed in terms of % by volume range from 58 to 44, 15 to 11 and 28 to 45% by volume, respectively; and wherein the apparatus is designed in such a manner that when inputting the parameters relating to the foregoing dry-etching conditions, directly or through a memory device of a computer, to the sequencer and then starting the dry-etching process, the dry-etching is automatically carried out under the foregoing dry-etching conditions.
According to a seventh aspect of the present invention, there is provided a dry-etching apparatus which comprises an etching chamber, a transport chamber, a substrate cassette bed and a sequencer for establishing dry-etching conditions, wherein four electromagnets each comprising a square-shaped ring-like coil are provided on the outer side of the etching chamber, two each of these electromagnets being opposite to one another and making a pair, these electromagnets being so designed that when applying a low frequency current which is 90 deg. out of phase thereto, the combined magnetic field established by these two paired electromagnets can rotate in a plane parallel to a substrate at a frequency identical to that of the low frequency current, an RF electrode and an opposite electrode are disposed in the etching chamber, a transport robot for transporting the substrate is provided in the transport chamber, the transport robot being a two-joint robot having two knots, the tip of a transport arm thereof being able to undergo advancing, reciprocating and rotating motions due to the composition of rotational motions of a motor axis and these two knots within each horizontal plane, the robot thus transporting the substrate, wherein a metal thin film to be dry-etched is a chromium-containing half-tone phase-shift film consisting of a chromium film, a chromium oxide film, a chromium nitride film, chromium oxynitride film, chromium fluoride film or a laminated film thereof, wherein if an etching gas used consists of chlorine, oxygen and hydrogen gases, the relative flow rates of these gases as expressed in terms of % by volume range from 66 to 46, 17 to 11 and 18 to 41% by volume, respectively, or if an etching gas used consists of chlorine, oxygen and hydrogen chloride gases, the relative flow rates of these gases as expressed in terms of % by volume range from 58 to 44, 15 to 11 and 28 to 45% by volume, respectively, and wherein the apparatus is designed in such a manner that when inputting the parameters relating to the foregoing dry-etching conditions, directly or through a memory device of a computer, to the sequencer and then starting the dry-etching process, the dry-etching is automatically carried out under the foregoing dry-etching conditions.